This is currently a copy of Ben's Judge's Tower, so all credit where it's due.

The Judge's Tower, or Judge Tower, is a casual Magic format originally created to help Judges, and those aspiring to be one, to test their knowledge of the Magic: the Gathering rules, as well as game state awareness. Get ready to rack your brain, pass priority, and enjoy keeping track of a lot of things!

Like all things Magic, the Judge's Tower relies upon the Magic Comprehensive Rules as its foundation. The Judge's Tower modifies or adds to existing rules, though, to keep the play environment quick and fun. Or maybe stressful, depending on your point of view. These rules are all based on the original Judge's Tower rules.

1. Alterations to Comprehensive Rules
   100.1 Should the Comprehensive Rules conflict with the Judge's Tower Incomprehensive Rules, the Incomprehensive Rules take precedence.
   100.2 All players have an infinite life total. This life total can increase or decrease, but life loss will never reduce it to a finite total. Life totals cannot be set to specific numbers.
   100.3 Each player’s starting hand is equal to their judge certification level. Players who are not certified judges - including Rules Advisors - have a starting hand size of zero.
   100.3 Players may not choose to take mulligans during the start of the game. However, this does not prevent effects that take place “anytime you could mulligan” from occurring
   100.5 All players share a library. If multiple players must take actions involving the cards in their libraries they then do so in turn order, starting with the active player and moving in turn order.
   100.6 All players share a graveyard. All players own all cards in the shared graveyard. If an object, spell, or ability affects cards in “your graveyard” (i.e. Past In Flames), players other than the one who controls the effect ignore the effect.
   100.7 The owner of a card is the first player to have it enter their hand, enter the Battlefield, enter the exile zone, or be put on the stack. While a card is in a shared zone, every player is considered to be its owner.
   100.8 When a player activates an ability of an object, that player may not activate that ability again until the beginning of their next turn. (Recall that a permanent that changes zones becomes a new object.)
   100.9 If the game would enter an infinite loop of mandatory actions, then all objects involved in the loop cease to exist and the game continues as normal.
   100.10 Players may only win or lose the game according the the rules outlined in ICR 500. State based actions that would cause a player to lose the game do not occur, and effects cannot make players win or lose.
   100.11 All players are considered able to pay any and all costs involving payments of mana and life (unless a game object would modify this).

2. As Soon as Possible Rules
   200.1 Players may not pass priority if there is another legal game action that they could take instead. (ICR 100.8 prevents a player from getting stuck repeatedly activating the same activated ability).
   200.2 If a spell or ability requires a player to choose X, they must choose five. If they are unable to select five, they must select the next highest legal value.
   200.3 If a spell or ability allows a player to choose a number of targets, that player must choose the greatest possible number of legal targets.
   200.4 If a spell or ability allows a player to choose a number of modes, that player must choose the greatest possible number of legal modes.
   200.5 All creatures attack each combat, if able. All creatures block each combat, if able.
   200.6 If an effect gives a player an option to perform an action, typically using words such as “may” or “unless,” that player must perform that action.
   200.7 If a player has mana in their mana pool, that player must use that mana for the first cost it can be used for before relying upon ICR 100.11.
   200.8 Players must acknowledge triggered abilities they control as early as possible.

3. Order of Play Rules
   300.1 When a player can take multiple game actions, they may choose which action to take first.
   300.2 When a player can activate multiple abilities of a card, they must activate them in order from the last ability to the first on the card.
   300.3 Any abilities granted to a card by other effects are assumed to be placed on the bottom of the card.
   300.4 When a player has to make a choice between objects in the graveyard or library, that player must choose the object or objects closest to the top of that zone.

4. Ending the Game
   400.1 A Judge's Tower is a series of Magic subgames that ends when any player would draw a card from the shared library and no cards remain.
   400.2 If a player would violate a rule of the Judge's Tower ICR or the MCR, the object related to the violation is removed from the game and that player loses the game.
   400.3 If a player misrepresents free or derived information (as outlined in the MTR Section 4), that player loses the game.
   400.4 If a player would lose the rule due to ICR 400.2 or 400.3, they do not lose the game until another active player points out and explains the violation.
   400.5 If a player notices an error that they have made that others cannot (e.g. due to hidden information), that player is considered "another player" for the purposes of ICR 400.4.
   400.6 If only one active player remains, that player wins the current subgame and adds a point token to their command zone.
   400.7 Whenever a subgame ends, remove from play all cards in non-library zones, then begin a new Magic subgame with the remaining cards in the shared library.
   400.8 Whenever the game would end according to ICR 400.1, the player with the most point tokens in their command zone wins the game.

5. Shame Variant
   500.1 The Shame Variant rules allow for a faster game of Judge Tower by removing subgames and modifying some of the game ending rules in ICR 400.
   500.2 When a game of Judge's Tower begins, all players begin with a Shame Counter in their command zone, set to zero.
   500.3 Whenever a player would lose the game due to ICR 400.2-5, that player instead increments their shame counter by one. (Objects are still removed from play as normal.)
   500.4 Whenever the sum total of all Shame Counters meets or exceeds four times the number of players, the game ends.